The solar system is everything held by the Sun’s gravity. It began about 4.6 billion years ago when part of a cloud of gas and dust collapsed, spun itself into a disk, and lit a star at the center. The leftovers clumped into planets, moons, asteroids and comets. The Sun kept 99.86 percent of the material. Jupiter took most of the rest.

The Sun is an ordinary middle-aged star, a ball of hydrogen and helium 1.39 million kilometers across. About 1.3 million Earths would fit inside. Its visible surface is about 5,500 °C; the core, where hydrogen fuses to helium, runs near 15 million °C. Energy made in the core takes tens of thousands of years to work its way out, then 8 minutes and 20 seconds to reach us.

On December 24, 2024, NASA’s Parker Solar Probe flew within 6.1 million kilometers of the surface at about 692,000 km/h, the closest approach and highest speed of any spacecraft.

Mercury is the smallest planet and the closest to the Sun, with an 88-day year. With almost no air to hold heat, its surface swings from 430 °C by day to −180 °C at night. Venus is nearly Earth’s twin in size and nothing like it otherwise: a carbon dioxide atmosphere 92 times the pressure of ours keeps the surface near 465 °C, hotter than Mercury.

Earth is the only place known to have liquid water on its surface and life. Its Moon, 384,400 km away, likely formed from debris after a Mars-sized body struck the young Earth.

Mars is half Earth’s diameter, cold and dry, with air less than 1 percent as dense as ours. It has the largest volcano known, Olympus Mons, about 22 km high, and a canyon system, Valles Marineris, that would span the United States. Riverbeds and lake deposits show it was once wet. NASA’s Perseverance rover has been collecting rock samples in Jezero Crater since February 2021.

Beyond the asteroid belt the planets change character. Jupiter is 11 Earths wide and more than twice as massive as all other planets combined. Its Great Red Spot is a storm larger than Earth that has blown for at least 190 years. Among its 95-plus moons, Europa hides a salt-water ocean under its ice, and Io is the most volcanic body known. NASA’s Europa Clipper, launched in October 2024, is due at Jupiter in April 2030.

Saturn is the showpiece, its rings made of countless chunks of water ice. Its moon Titan has a thick atmosphere and lakes of liquid methane; Enceladus sprays water from a buried ocean into space. Uranus rolls around the Sun tipped on its side, and Neptune, 30 times farther from the Sun than Earth, has the fastest winds measured on any planet. Only one spacecraft, Voyager 2, has visited either, in 1986 and 1989.

Pluto, found in 1930 and reclassified as a dwarf planet in 2006, turned out to be anything but dull when New Horizons flew past on July 14, 2015: nitrogen glaciers, water-ice mountains and a heart-shaped plain. It is one of the largest members of the Kuiper Belt, a zone of icy bodies beyond Neptune.

Between Mars and Jupiter lies the asteroid belt, whose largest member, Ceres, is 940 km across. In September 2023 NASA’s OSIRIS-REx returned 121 grams of the asteroid Bennu to Utah. A year earlier the DART spacecraft deliberately struck the small asteroid Dimorphos and shortened its orbit by about 32 minutes, the first test of deflecting an asteroid.

Comets are dirty ice from the cold outer reaches. Far beyond the Kuiper Belt, the Oort Cloud is thought to hold trillions of them out to perhaps 100,000 times Earth’s distance from the Sun, nearly halfway to the next star.

There are several edges. The planets end at Neptune, 30 AU out. The Sun’s wind of charged particles blows a bubble, the heliosphere, that Voyager 1 left in August 2012 at about 121 AU. The Sun’s gravity holds the Oort Cloud a thousand times farther still. By that measure, both Voyagers will be inside the solar system for tens of thousands of years.

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